Silencing circuits for radio receivers



Dec. 7, 1937. J J LAMB SILENCING CIRCUITS FOR RADIO RECEIVERS Filed April 18, 1955 3 Sheets-Sheet l .RB Wm w 1?. mOI. v I M m R E H WSYM U M... THWB m 4 A F 4 J :M I I ulmlnlmlnlmlal De c. 7, 1937. J 1 LAMB W 2,101,549

SILENCING CIRCUITS FOR RADIO RECEIVERS INVENTOR UAMESUP/ AMB BY QQM.

ATTORNEY Dec. 7, 1937.

J. J. LAMB 2,101,549

SILENCING CIRCUITS FOR RADIO RECEIVERS Filed April 18, 1935 s Sheets-Sheet 5 SCOND DTCTOR PRESELECTOR AND RADIO FREQUENCY 'AHPL/F/[R AUD/O AMPLIFIER E $1 i E i J:-

v I i v A W? SPEAKER 1 FIRST 027E670)? \ilLE/vcvA/s cvecu/rs i INTERMEDIATE FREOUENC; OSCILLATOR AMPLIFIER FIG UFQE 0 F 5. gw l Pi WW THY b MM d 3 Q 0 -GR/D v04 rs -c FIGURE VI I: E Y 3 INVEL N TOR JANE, S d. AME

AT T QR Patented Dec. 7, 1937 PATENT. OFFICE SILENOING CIRCUITS FOR RADIO RECEIVERS James J. Lamb, West Hartford, Conn, assignor, by mesne assignments, to Alan N. Mann, Scarsdale, N. Y., trustee Application April 18, 1935, Serial N0, 16,961

25 Claims.

My invention relates to radio receivers. More specifically it is a circuit for silencing undesirable noises inv radio receiving circuits. One of the objects of my invention is to limit undesired voltage impulses which may cause disturbing noises in radio receivers. Another object is to completely silence an undesired voltage impulse which exceeds the voltages induced by, desired signals. A further object is to completely silence the radio receiver for a very short time simultaneous with the occurrence of static or other noise producing effects. Another object is to automatically adjust the silencing circuitsto operate at varying levels depending upon the voltages induced by incoming carrier currents.

, I am aware of the proposal of 'Scott-Taggart in British Patent 185,133 of 1922. This was a step in the direction of silencing rather than phase balancing or limiting static impulses. The teachings of that patent do not go far enough to be of real utility for any but the strongest undesired impulses. My invention will operate on small undesired impulses which are but slightly in excess of the desired signals. The circuits of my invention will be illustrated in the accompanying drawings.

Figure I illustrates one form of my invention applied to an intermediate amplifier of a supertrated in Figure heterodyne.

Figure II represents my invention with automatic control of the silencing circuit.

Figure III represents another circuit of my invention in which a frequency conversion of the poise producing voltage is employed.

, Figure IV illustrates a modification of my invention in which a separate duo-diode rectifier is employed.

Figure'V is a schematic arrangement showing the noise'reducing circuits of my invention applied to a superheterodyne with automatic volume control.

Figure VI is a grid voltage-plate current curve illustrating the operating characteristic of my invention.

In the accompanying drawings similar reference numeralswill be used to represent similar elements in the various figures. In Figure I, an incoming'signal and noise, or static source of high frequency currents is represented as the terminals i, and 2. These term'inals are connected -to the tuned circuit consisting of inductance 3 and capacity 4. The tuned circuit may be part of an intermediate frequency amplifier as illus- V,'or a tuned radio trequency amplifier. c

Coupled to inductance 3 are two secondary inductances 5, and 6, which are tuned respectively by capacities 9 and III. The three circuits, 3-4, 5-9, and 8-! are preferably resonant to the same frequency, which, in the case of an intermediate frequency amplifier, may be 465 kilocycles', by way of example. The tuned circuit 5-9 is grounded at H and its'high potential terminal is connected to the outer control grid i2 of multi-grid tube IS. The anode is connected to a resonant circuit comprising inductance l6 and variable capacity H. I V

The tuned circuit IG-l'l is connected to the positive terminal iii of B battery IS. The battery I9 may be by-passed by a capacitor 20. Inductance I6 is mutually coupled to inductance 2| across which is shunted capacity 22. The combination 2l-22 is tuned to resonance. The terminals 23-14 represent the detector, audio frequency amplifier,- and loud speaker illustrati-ed in Figure V.

The screen grid 25 and an auxiliary grid 26 are connected to a suitable positive potential point on B battery l9, which is by-passed by a capacitor 21. The B battery I 9 is grounded at 28. The cathode 29 is grounded through self-biasing resistance 30 and by-passed by capacity 3i The remaining grid 32 of tube I3 is. the inner control grid, and its function will be described in connection with silencer tube 33. Inner control grid 32 is in the same electronv path as outer control gridlIZ.v

For simplicity of illustration the cathode heating battery ofthe various tubes in the accompanying drawings has been omitted. It should be understood that the operating potentials may be secured from batteries or rectified alternating currents. In normal operation the incoming signal representing'currents across the terminals =l2 will be amplified in the conventional manner by tube i3 and appear in amplified form across the terminals 23-44 whenno excessive undesired impulses are present:

I shall now describe the circuits and operation of tube --33. One terminal or the resonant circuit 6-i0 is connected-to the grid 34 01 tube 33; the other terminal-of the resonant circuit 6-! is connected to'the slider on potentiometer 35. The potentiometer 35 is connected across C battery 36 whose positiveterminal is grounded. The potentiometer and battery are by-passed by capacity 3|. The cathode 331s grounded. Con-- nected to the cathode 38 1s suppressor grid 39. The screen grid '40 is connected to the screen grid 25. The anode M 'is connected to one teracross the terminals I--2.

minal of inductance 42. The other terminal of the inductance 42 is connected to the positive terminal I8. Capacity 43 is shunted across the inductance 42. The elements of inductance 42 and capacity 43 are preferably adjustedto resonance.

The inductance 42 is mutually coupled to secondary inductance 44. This inductance is shunted by a capacity and preferably tuned to resonance. One of the terminals of 45 is connected to anode 48. The other terminal of 45 is con-- nected to anode 41.

The center point of inductance 44 is grounded through resistance 48 which may have a value of the order of several thousand ohms to two megohms. Resistance 48 may be by-passed or used without by-passing. If by-passed, the capacity 49 must be of a small value. In any event the combination of resistance 48 and capacity 49 must be chosen so that its time constant, which is determined by the product of the capacity times the resistance, is extremely low; i. e., 'of short duration. I prefer a time constant not longer than one ten-thousandth (1/10,000) of a second. The center tap of the inductance 44 is also connected through radio-frequency filter impedance to the control grid 32 to tube I3.

Having described the essentials of the circuit and the normal operation of tube I3, I shall now describe the function of tube 33. The control grid 34 is biased to a negative potential by means of the potentiometer 35 which is just sumcient to render tube 33 inoperative for the voltages induced in the resonant circuit 6I0 by normally desired signals representing currents impressed If a sudden burst of electrical noise or static representing voltages in excess of the signal is induced across the resonant circuit 6-I0, that voltagewill exceed the bias on control grid 34 and will'be amplified in the anode circuit 4|, 42-43. The amplified signal in the anode circuit 42 is induced in the secondary inductance 44. It is rectified by thefull wave diode elements 33, 46, and 47.

The rectified current, because of the full wave rectification, appears across the resistance 48 as the envelope of pulsating current equal to twice the fundamental frequency. The grounded end of the resistance 48 is at positive potential while the mid-point or center tap of the inductance 44 is negative. The negative voltage thus developed is applied to the inner control grid 32 of tube I3. The amplified andrectified potential is so high that the tube l3 will immediately be biased to cut-off and effectively disabled. Simultaneously the undesired voltage impulse, along with the desired signal voltagesjwill be impressed on outer grid l2 but since tube I3 is disabled by grid 32, no voltage representing either desired signals or undesired static will appear across the terminals 2324. The cut-off or silencing action of the grid 32 will be most effective if the grid 32 has a greater control action on the anode currents of tube I3 than the grid I2.has on the same anode currents. It should be understood that complete cut-oil is not essential in every instance. Greatly diminished amplification in tube I3, for the instant corresponding to the static disturbance, will greatly improve reception.

Since the averagev electrical noise or static impulse'is a damped wave of extremely short duration, I have found that the action of the above described silencing operation has little or no observable effect on broadcast telephonic signals. Such effect may be illustrated in Figure VI, In-

,rectified voltage to control tube I3.

.coming desired carrier voltages. propose to automatically regulate the threshold coming modulated carrier voltages impressed on the signal grid are represented as A. The normal effect of A is represented by amplified anode circuit currents such as B. An incoming excessive static impulse is represented as C. The

effect of C on the anode currents is shown as D.-

The disturbance E is not quite sufficient to cause cut-off of the amplifier tube anode current but the gain of the tube is instantaneously and temporarily reduced as represented at F.

In general D and -F are of such short duration, and the signal representing currents of such complex nature, that the intervals D and F are not detected by the listener. If the undesired impulse is of longer duration, it will silence the radio receiver but that is preferable to excessive, loud static noise crashes. If the impulses C or E are of recurrent nature but of very short duration, the receiver will be effectively silenced without undesirable crashes or modulation effects.

In carrying the system illustrated in the several figures into practice certain precautions should be observed. The gain or amplification through tube 33 must be high to provide suificient This high gain tends toward instability. The exact balancing of resonant circuit '4445 about the center point of the inductance 44 is very helpful as it of itself tends to balance or neutralize feedback eifects. The double frequency component of the rectified pulsations across the resistance 48 is partially filtered by the resistance 50 and less apt to start self oscillation than if the control voltage were of the fundamental resonant frequency.

Since the whole silencing action is dependent upon the thermionic action of tubes I3 and 33, it is apparent that any stray coupling other than this electronic coupling by thermionic action between the terminals I-2 and the second terminals 23-24 will tend to defeat the silencing action. To this end the'input circuits of tubes I3 and 33 must be carefully shielded from their output circuits. This is true both within the tube and without. Complete magnetic and capacity shielding represented by the enclosures of Figure V is preferred. The tubes should be carefully chosen, by way of example, tube I3 may be an RCA type 2A7 and tube 33 may be an RCA type 2B7. In place of tube 33, a separate screen grid amplifier and separate duo diode may be employed, as illustrated in Figure IV.

In the case of Figure I the cut-off or silencing point must bemanually adjusted by means of the potentiometer 35 to determine the threshold operating point of tube 33 with respect to the in- In FlgureII, I

silencing point by an automatic gain or volume control action. In Figure II the B voltage source connected between the terminals 5I52 is shunted by resistances 53 and 54. The cathode 38 is biased to a suitable positive potential by a contact 55 on the potentiometer 53.

The inductances 5 and 6 are connected by capacity 56. The combination of the indu'ctances 5,

and tuned to resonance. The inductance 5- is grounded at 58. Inductance 5 is grounded for radio frequency currents by capacity .56. The

resistance 59 is connected to a source of auto- '6 and the capacity 56 is shunted by capacity 51 Lil approximately by means of the contact 55, the A. V. (i/(automatic volume control) voltage will bias the grid 34 to the proper threshold operating potential with'respect to the incoming carrier currents because the A. V. C. voltage is derived from the carrier and is proportional to it. The remaining elements of Figure II are similar to the corresponding elements of Figure Iand perform the same function. r

The structure electrically interposed between the input circuit (represented by the terminals i and 2) and the output circuit (indicated by the terminals 23 and 24) and including both tubes i3 and 33 and the connections between them, can be considered as a unitary limiting device which is connected to the input circuit and is adapted to transmit to the output circuit currents derived from the input circuit. It is to be remembered that grid 34 is normally biased negatively beyond the cut-off point by the setting of element so that tube 33 is inoperative, and this tube will come, into operation only when the voltage receiveti ,from the input circuit (such as a noise impulse) overcomes this bias sufficiently to make the anode circuit conductive. When this occurs the rectified output of tube 33 serves to reduce or cut off the output of tube l3. Hence, the maximum voltage that can be transmitted by tube 83 is limited and determined by the bias of tube 33. In Fig. 11, the negative bias applied to grid 34 is derived not only from the setting of element 55 but this negative bias is increased by the voltage supplied through resistance 59. This voltage is proportional to the carrier currents since it is 0 derived from the same source as the voltages used for automatic volume control. In contradistinction to the ordinary use of A. V. Czvoltages, its effect here is not to control the gain of tube 33 but to control the point atwhich this tube changes from its inoperative condition to its' operative condition. That is, in the device we are here considering, the action of this voltage is to control the maximum output of the limit- .ing device in such a way that when the carrier current increases, the maximum value of the currents which may be transmitted .to the output circuit is proportionately increased. As is customary with A. V. C. voltages, this control voltage should be sluggish in its changes so that the changes in the maximum value of the current that can be transmitted by the limiting device will vary with changes in the carrier current, but will not vary appreciably with the brief changes in intensity resulting from noise impulses or the like. This effect is particularly true where, as

here, the control voltages are derived from a source positioned beyond the limiting device.

We can quite properlyl consider this limiting device taken in its entirety as being in effect a gate. or valve. long asthe current received from the input circuit does not exceed a predetermined value, the gate remains open sufliciently so that such current passes through unimpeded. However, if becauseof some noise impulse the current exceeds this predetermined value, the gate prevents the passage of the excesscurrent. For such a device to be most effective, the gate must be adjusted so that the carrier current can just pass through with a very slight excess thus keeping noise to a minimum, yet not interfering with the program. To-get this result with the structure of Fig. I demands manual setting of the potentiometer 35 for each change in carrier current. However with the arrangement here input is correspondingly. increased.

' of kilocycles.

potentiometer 53.

shown in Fig. II, a change in the carrier current automatically changes the threshold of tube 33 so that as the carrier current increases, the gate opens wider and increases the maximum value of the voltages which can-be transmitted by the limiting device to the output circuit.

In certain arrangements I have found that very high gain is essential in the silencing circuits. With increasing gain, the feedback from the output of the silencing system through the various circuits, couplings, and capacities to the Special means are necessary, in these cases, to prevent sustained oscillation. In Figure III, instead of tuning the output of tube 33 to the incoming frequency, I show'an arrangement in which a local oscillator 50 is coupled through inductance G-l to the grid 34. The output circuits 42-43 and 44-45 may be tuned to 175 kilocycles. In case the incoming frequency is 465 kilocycles, the local oscillator 60,may be tuned to 290 kilocycles. The combination of 465-kilocycle incoming voltage and 290-kilocycle oscillator voltage within tube 33 will produce a resultant voltage of a frequency of 175 kilocycles in 42-43 and 44-45. The rectifying action may now take place in-a single diode but I prefer the duo-diode. If the full wave rectifier is used, the pulsating rectifier currents will have a frequency of '350 kilocycles and residual fundamental currents a frequency With the input circuits tuned to 465 kilocycles and the output circuits of tube 33 tuned to 1'75 kilocycles, the disturbing feedback efiects may be avoided. In Figure III the A. V. C. voltage for the bias of grid 34 may be used as described in connection with Figure II.

In Figure IV,-the general arrangement of the preceding circuit diagrams is followed. In the case of Figure IV the rectifier ill is a separate tube from the amplifier 33. In this particular arrangement a single resonant circuit 5-9 is used the filter. network which consists of radio frequency choke l2, resistance 13 and by-pass condenser 14. The cathode 38 of tube 33 is selfbiased by resistance 15 which is by-passed. by a capacity 15.

The duo-diode anode elements 46 and 41 are connected to the inductance 44. The'cathode i'l of tube 10 is connected to the variable slider 18 on The by-pass condenser i9 is connected from cathode 11 to ground. The slider 18 is used to bias the cathode positive with respect to the anodes 45 and 41, to determine the normal threshold voltage which will be rectified by the rectifier i0. Since the anodes are both negative relative to the cathode, by the amount of the voltage drop between the terminal! and the slider I3, no current will fiow in the rectifier until that .voltage drop is exceeded by the voltages induced iii the resonant circuit 44-45 by the currents flowing in the amplifier output circuit 42-43. The center point of the inductor is connected to the resistance 48 and the resistance 48 in turn is connected to ground. A small capacity 49 may be shunted across resistance 48. The

capacity 49 is of a very small value to keep the time constant of resistance 48 and capacity 49 extremely low. The connection from the resistance 48 to grid 32 of tube l3 may include a reactance or radio frequency choke 80. The inductance 80 is chosen so that the combination of the choke 80 back currents and thus tends to improve the stability of the circuit.

If the radio frequency transformers happen to have a very high ratio of reactance to resistance,

their damping: is lessened and their selectivity characteristic is very sharp. This sharpness is not wholly desirable within silencing circuits of the type just described. In such cases it is preferable to dampen the circuits by a shunt resistance such as [5 or l5. These resistances may be of the order of 100,000 ohms or less and may be employed in any of the various circuits in which sharply tuned transformers are used.

Thus I have described circuits in which parallel paths are employed. One path is the normal amplifying path for the currents to be amplified. The other path is operative only for undesired impulses of greater than amplitude than the signal. The undesired impulse is amplified in the second path, rectified, and the rectified impulses are used to prevent transfer through the normal path wiihout affecting the stability of the circuits.

I claim:

1. A radio frequency amplifying system comprising two parallel paths: the first, a thermionic amplifier including an input circuit, an output circuit, and a control grid electrode; the second, a thermionic amplifier, including an input circuit, an output circuit, and a rectifier coupled to the last mentioned output circuit; means connected in the second path to prevent its operation for desired signal representing voltages; means connected in the rectifier circuit to derive pulsating voltages from rectified currents; and a connection including a. series impedance from said rectifier to said control grid electrode so that the first path is rendered ineffective for voltages exceeding said desired signal voltages.

2. A radio frequency amplifying system comprising two parallel paths: the first, a thermionic amplifier including an input circuit, an output circuit, and a, control grid electrode; the second, a thermionic amplifienincluding an input circuit, an output circuit, and a full wave rectifier coupled to the last mentioned output circuit; means connected in the second path to prevent its operation for desired signal representing voltages;

3. In the intermediate frequency amplifier of a superheterodyne receiver two parallel paths: the first, a screen grid thermionic amplifier including an input circuit, an output circuit, and a'control grid electrode; the second, a screen grid thermionic amplifier, including an input circuit, an output circuit, and a rectifier coupled to the last mentioned output circuit; means connected in the second path to prevent its operation for desired signal representing voltages; means operating on a brief time constant connected in the rectifier circuit to derive pulsating voltages from rectified currents; and a connectioncomprising a series impedance whereby the pulsating voltage from said last-mentioned means can be applied to said. control grid electrode and oscillation pre vented.

4. In the intermediate frequency amplifier of a superheterodyne receiver two parallel paths: the first, a screen grid thermionic amplifier including an input circuit, an output circuit, and a control rid electrode; the second, a screen grid thermionic amplifier, including an input circuit, an output circuit, and a full wave rectifier coupled to the last mentioned output circuit; means connected in the second path to prevent its operation for desired signal representing voltages; means operatingon a' brief time constant connected in'the rectifier circuit to derive pulsating voltages from rectified currents; and a connection comprising a series impedance from said rectifier to said control grid electrode whereby pulsating voltages from said last-mentioned means can be. applied to said control grid electrode without causing oscillation.

5. In the intermediate frequency amplifier of a superheterodyne receiver two parallel paths: the first, a screen grid thermionic amplifier including an input circuit, an output circuit, and a control grid electrode; the second, a screen grid thermionic amplifier, including an input circuit, an output circuit, and a rectifier coupled to the last-mentioned .output circuit; means connected in the second path to prevent its operation for signal representing voltages; means connected in the rectifier circuit to derive pulsating voltages from rectified currents; and a series connected impedance connection from said rectifier to said control grid electrode so that static representing voltages exceeding said signal voltages are attenuated in said first path.

6. In the intermediate frequency amplifier of a superheterodyne receiver two parallel paths: the first, a screen grid thermionic amplifier including an input circuit, an output circuit, and a control grid electrode; the second, a screen grid thermionic amplifier, including an input circuit, an output circuit, and-a full wave rectifier coupled to the last mentioned output circuit; means connected in the second path to prevent its operation for normal signal representing voltages; means connected in the rectifier circuit to derive pulsating voltages from rectified currents; and a series connected impedance connection from said rectifier to said control grid electrode for biasing same to diminish the amplification of the first mentioned thermionic amplifier, whereby the first path is rendered ineffective for voltages exceeding predetermined normal limits.

'7. A radio frequency amplifying system including a source of incoming signal and static currents; a thermionic amplifier coupled to said source for amplifying said currents, and a control grid included in said amplifier; a second thermionic amplifier also coupled to said source and including a rectifier; means to prevent said second thermionic amplifier and rectifier from responding to normal incoming signal currents; means for impressing upon said control electrode voltages determined by said rectifier from static currents whose value exceeds the normal incoming signal currents; and a sourceof direct input control voltage for the second mentioned amplifier derived from said incoming signal currents.

8. A device of the character described in claim 3, including, a source of local oscillations of substantially constant frequency lower than the frequency of said incoming signal currents to convert the frequencies in the amplifier of .the second path to a value differing from the frequency of said incoming signal currents.

9. A device of the character described in claim I 7 including, a source of local oscillations to con-' vert the incoming signal currents in the second and including a rectifier; means to prevent rectification in said rectifier of normal incoming signal currents; means including resistance and capacity whose time constant is of the order of one ten thousandth of a second for deriving a voltage from said rectifier from currents of .a,

value exceeding said'signal currents; and a connection from the last mentioned means for linpressing a negative voltage on said control grid electrode.

12. In a 'device' of the character of claim 7, a

- resistance shunted across said signal current am- 25v plifying path. a

13. In a device of the character of claim 7, a resistance shunted across said first mentioned amplifier path and a second resistance shunted across said second amplifier path.

14. A radio frequency system including a source of incoming signal and static currents, a thermionic amplifier coupled to said source for amplifying said currents and a control grid included in said amplifier, a second thermionic amplifier also coupled to said source, a rectifier associated with said amplifier, means to'prevent rectification of normal incoming signal currents in said rectifier, means including resistance and capacity for building up a voltage from currents from said rectifier of a value exceeding said signal currents and a connection from the last amplifying normal carrier currents in said first amplifier, amplifying excessive impulses in said second amplifier, rectifying the full wave of the output of the second amplifier to produce a rectified current of double the frequency of the impulses transmitted to the first. amplifier, and

building up with said rectified current a potential negative with respect to the cathode of the first amplifier and applying said potential to the control grid of the first amplifier whereby the mutual conductance of the said amplifier is reduced when the second amplifier is functioning.

16. The method of amplifying carrier currents and preventing amplification of excessive impulses by means of a thermionic amplifier for normal signals connected to an input circuit and comprising a control grid and a second amplifier for excessive impulses arranged in parallel connection to the same input circuit but normally biased to prevent its operation; which comprises amplifying normal carrier currents in said first amplifier, amplifying excessive impulses in said second amplifier, rectifying the full .wave of the output of the second amplifier-to produce a rectified current of double thefrequency of the impulses transmitted to the first amplifier and applying such current to the control grid of the first amplifier so as to cause the mutual conductance of said amplifier to be reduced when the second amplifier is functioning.

17. A radio frequency amplifying system comprising two parallel paths; the first a thermionic amplifier including an input-circuit, an output circuit and a control grid electrode; a second thermionic amplifier including an input circuit,

an output circuit and a rectifier coupled to the last-mentioned output circuit; means connected in the second path to prevent its operation for desired signal representing voltages; means connected in the rectifier circuit to derive pulsating voltages from rectified currents; a connection from said rectifier to said control grid electrode so that the-first path is rendered ineffective for voltages exceeding said desired signal voltages, and a source of local oscillations to convert the incoming signal currents in the second path to a frequency different from the frequency of said incoming signal currents.

- 18. A device for receiving carrier currents and automatically preventing the reception of excessive impulses, comprising an input circuit adaptedfto transmit carrier currents and excessive impulses, an output circuit, limiting means connected with such input circuit adapted to transmit to such output circuit currents received from the input circuit limited approximately to a predetermined maximum value, control means adapted to be actuated by said carrier currents for developing direct current voltages proportional to the carrier'current intensities, and means for applying such voltages to said limiting means to increase such maximum value of the currents which may be transmitted to said output circuit by said limiting means to an extent proportional to the value of the carrier current.

19. A device as specified in claim 18, in which the said control means derives current from a source operatively connected with said output circuit.

. 20. A device as specified in claim 18, in which the said control means derives current from a source operatively connected with said output circuit and which also includes means for biasing said limiting means to maintain the current in said output circuit during reception at a positive value in excess of zero. 1 l

21. A structure as specified in claim 18, which further includes a thermionic amplifier connected in advance of said limiting device and having an output circuit which serves as the input circuit of the said limiting device and in which said control means derives current from a source con.- nected with the output circuit of said limiting means, and which further includes means-for deriving current from a source connected with said output circuit of the limiting means and applying the same to control the amplification of said thermionic amplifier.

i 22. The combination with a limiting device for radio circuits comprising a pair of radio tubes, means whereby the output of one of said tubes serves to limit and control the maximum value of the output of the other of said tubes and means for establishing a threshold for said controlling tube, of means for deriving direct current voltages proportional to carrier currents transmitted to said controlled tube and means for applying such direct current voltages to the grid of said controlling tube to modify the threshold thereof, so constructed and arranged that as the carrier current increases, the said threshold will be modified to maintain said controlling tube inoperative for voltages within the range of normal modulation of the carrier current.

23. The combination with a limiting device for radio circuits comprising a pair of radio tubes, means whereby the output of one of said tubes serves to limit and control the maximum value of the output of the other of said tubes and means for creating a negative bias for the grid of said controlling tube .to establish athreshold for the same, of means for deriving direct current voltages proportional to carrier currents transmitted to said controlled tube, and means for applying such direct current voltages to modify the bias of the grid of said controlling tube so that as the carrier current increases, the bias of the grid of the controlling tube will be maintained at such value that said controlling tube will be kept inoperative for voltages within the range of normal modulation of the carrier current.

24. A device for receiving the normal modulation of carrier currents and automatically preventing the reception of excessive impulses, comprising an input circuit, an output circuit, limiting means connected with such input circuit adapted to transmit to such output circuit currents received from the input circuit, such limiting means comprising means for limiting the transmission of impulses received from the input circuit and rising above a determinable maximum value and being adapted to transmit currents below such value at substantially uniform normal efilciency, means adapted to be actuated by carrier currents received by such device to develop direct current voltages proportional to such carrier current intensities, and means for applying such voltages to said limiting means to raise and lower the said maximum value of currents transmitted at normal efliciency relative to the car; rier current so as to permit the transmission by said limiting means of the normal modulations of the carrier current at approximately normal efficiency and to maintain such maximum value low enough substantially to limit brief impulses of substantially greater intensity.

25. In a device for receiving the normal modulations of carrier currents and automatically preventing the reception of excessive impulses comprising an input circuit, an output circuit, limiting means connected with such input circuit comprising a tube adapted to transmit to such output circuit currents received from the input circuit and further comprising control means operating on a brief timeconstant for preventing the transmission by said tube of currents, above a determinable level while permitting said tube to transmit currents below sucl level at substantially normal efiiciency, means for adjusting said control means whereby the level at which such control means will act may be varied, means operating on a sluggish time constant adapted to be actuated by carrier currents received by the device for developing direct current voltages proportional to the carrier current intensity, and means for applying such voltages to said control means to modify the level at which such control means will act so that such level will be raised and lowered with and in relation to increases and decreases of the carrier current so as to permit-the transmission by said tube of normal carrier currentmodulations at approximately normal efficiency and .to maintain such level low enough substantially to limit brief im-- pulses of substantially greater intensity.

JAMES J. LAMB. 

